1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an outer loop power control apparatus for setting a target energy-to-noise ratio (Eb/No) adaptively according to channel condition, and a method thereof.
2. Description of the Related Art
A CDMA (Code Division Multiple Access) mobile communication system is comprised of Node Bs, RNCs (Radio Network Controllers), and UEs (User Equipments). Each Node B performs downlink TPC (Transmit Power Control) and uplink TPC to minimize interference between UEs within its cell and interference from other Node Bs. The downlink TPC controls the power of a downlink signal transmitted from the Node B to a UE, and the uplink TPC controls the power of an uplink signal transmitted from the UE to the Node B. The downlink and uplink TPCs are commonly performed by open loop power control, closed loop power control, and outer loop power control.
Those power control methods will be described below.
(1) Open Loop Power Control
A UE measures the propagation loss of a particular downlink channel signal, for example, a P-CCPCH (Primary Common Control Physical CHannel) signal received from a serving Node B and controls its uplink transmission power according to the propagation loss measurement, to thereby allow the Node B to receive an uplink channel signal reliably.
The P-CCPCH delivers information about the Node B and system information (SI) to UEs within the service area of the Node B. The P-CCPCH is transmitted with a constant transmission power and information about the transmission power of the P-CCPCH is broadcast to the UEs. Thus, the UE can measure the propagation loss during the P-CCPCH transmission based on its transmission power. Hence, an initial target SIR (Signal-to-Interference Ratio) is set by the open loop power control.
(2) Closed Loop Power Control
The UE measures the strength, that is, an SIR of a downlink channel signal received from the Node B. If the SIR is less than a target SIR, the UE transmits to the Node B a TPC command requesting an increase in the transmission power of the downlink channel signal. Alternatively, if the SIR is equal to or greater than the target SIR, the UE transmits to the Node B a TPC command requesting a decrease in the transmission power of the downlink channel signal. The Node B then controls its transmission power according to the TPC command received from the UE.
(3) Outer Loop Power Control
The above-described closed loop power control is based on an SIR. However, the quality of a radio channel signal is evaluated based on an FER (Frame Error Rate) rather than the SIR in an actual mobile communication system. The FER indicates an error rate limit for a digital signal required to provide good voice quality. The FER is closely related to user satisfaction with communication quality. Therefore, an FER at which the quality of a radio channel signal is maintained at an desirable level, that is, a target FER, is set according to the characteristics of a mobile communication system.
If a power control is performed in the closed loop power control method alone, an actual FER measurement varies with channel environment despite a same SIR. As a result, the FER measurement is above or below the target FER, leading to an inefficient use of the entire system capacity. That is, the relation between the SIR and the FER changes irregularly due to external factors including a channel environment and a velocity of the UE.
In this context, there is a need for a power control that maintains the target FER for use in the closed loop power control by changing the target SIR adaptively according to the channel condition. This power control is an outer loop power control. The outer loop power control method changes the target SIR used in the closed loop power control adaptively according to the channel condition in order to maintain a particular performance characteristic such as the target FER.
In the CDMA mobile communication system, if the outer loop power control is adopted, the quality of a received frame is determined by CRC (Cyclic Redundancy Check). If it turns out that the frame has errors, a target Eb/No is increased by a predetermined increment unit up_step. On the contrary, if the frame has no errors, the target Eb/No is decreased by a predetermined decrement unit down_step. The target Eb/No is used in the concept of the target SIR herein.
The increment unit up_step and the decrement unit down_step are in the relation expressed asup_step=K×down_step   (1)The variable K is calculated by
                    K        =                              1                          reqd              ·              FER                                -          1                                    (        2        )            where reqd.FER is a required FER for a received frame.
As a plurality of frames are successively received during communications, the FER eventually converges to the reqd.FER as noted from Eq. (1) and Eq. (2). As stated above, reqd.FER is a target FER for a corresponding channel. For example, if reqd.FER is 10−2 and K×down_step is 0.5[dB], K is 99 by the above equations. When a received fame has errors, its transmission power is increased by 0.5[dB] and if the frame has no errors, the transmission power is decreased by
            0.5      99        ⁢                  [    dB    ]    .
In the outer loop power control, a received frame is CRC-checked as stated above. If the frame has no errors in the CRC check, a target Eb/No [dB] is decreased by a relatively small decrement unit down_step. On the contrary, if the frame has errors in the CRC check, the target EB/No [dB] is increased by a relatively great increment unit. An FER in an actual channel environment converges to a target FER by adjusting the EB/No decrement unit down_step based on the Eb/No increment unit up_step.
Despite the advantage of stable maintenance of received channel signal quality in a bad channel environment, the outer loop power control causes excess power consumption because transmission power is decreased a plurality of times, each time by a small decrement unit in a good channel environment such as indoors and increased by an increment unit greater than the decrement unit in the bad channel environment. This limitation becomes serious in a mobile communication system strictly requiring maintenance of a target FER, especially in the good channel environment.